A unique and curious new species of Dracula ants - so-called because they feed on the blood of their young - has been discovered in the rotting leaf litter of a Madagascar forest, and scientists say it will force them to rethink the entire path of ant evolution.

To entomologist Brian Fisher, who discovered the insects, the find may well prove as significant for the insect world as discoveries of humanity's fossil ancestors in Africa, which offered new insights into human evolution.

His find, Fisher says, could offer a crucial key to understanding how ants have evolved and diversified from their ancestors, the wasps, tens of millions of years ago.

Fisher, the assistant curator of entomology at the California Academy of Sciences in San Francisco, has spent five years in Madagascar searching for ants, including the new species, unique members of the ant genus Adetomyrma.

Ant study can be a risky business at best. In the course of his many years hunting ants throughout Africa and Madagascar, Fisher has suffered just about every tropical disease. "They've used medicines on me that you can't even find in America," he said.

Slender, bearded and filled with excitement, Fisher the hunter has already discovered 600 new ant species in Madagascar - all new to science, and 74 of which are members of the genus Strumagenys.

Only weeks ago, Fisher plunged his scarred arm into a rotten log and uncovered the first colony of red-orange Dracula ants, complete with their yellow queen and white larvae. He recognized instantly that they were different from all the other species he had discovered.

"There they were, crawling over me and stinging my hand, and I was crazy with excitement and joy," he said. "I was, like, 'oh my gosh, are these what I think they are?' The driving force for all of us in science is the thrill of discovery, and here was my biggest thrill of all right there."

Madagascar, an island off the coast of East Africa, is one of the world's centers of unique biodiversity. It split from Africa some 180 million years ago, and its isolation has enabled scores of plants and animals to evolve into new species.

Only in recent years has Madagascar become prey to forces of development. Its plant and animal life - including the primitive primates called lemurs - are threatened as never before: The human population is soaring, native trees are being harvested and roads are carving up the landscape.

But the ants are still there, although as Fisher laments, "They are relics of the earliest days of evolution, and if we don't collect some to study now, in another 10 years they'll be gone."

The new species has unique characteristics. They are the only ants that have a rear third of the body, called the gaster, connected directly to the middle section, called the thorax. That connection is almost identical to the bodies of wasps, from whom these ants are believed to have descended some 70 to 80 million years ago.

In all other ants, Fisher explained, a series of slender, constricted segments link the gaster and thorax. That segmented waist endows them with greater flexibility - perhaps to escape from predators in tight spaces. The new Madagascar species, with far fewer natural predators, may never have needed to evolve a more flexible body.

While most ants are noted for their "social food transfer" behavior, in which the workers share solid food with each other and carry it to the solitary queen in each colony, members of Fisher's new species are, as he puts it, "nondestructive cannibals."

To humans, they would seem savages indeed. Like vampires, the workers must prey on the larvae in their own nursery, scratching the skin of the larvae until they bleed, then chewing on the young, gorging on the blood and regurgitating the blood to feed the sedentary, wingless queens.

"The larvae aren't killed," Fisher said, "but when you look at them closely in the colony, you can see that they're all scarred."

Both in their vampire-like social behavior and their thickened bodies, Fisher's ants seem to lie somewhere between more primitive ants and their ancestral wasps on one side, and more recently evolved ants on the other.

In 1993, Philip Ward, an entomologist at the University of California at Davis, was exploring in Madagascar and found a cluster of worker ants that also bore the unique body shape Fisher found a month ago. But he could go no further in his research because he found no larvae, no males and no queen. He simply called them "enigmatic" in a scientific journal report he published.

Now the enigma is ever more puzzling, Ward said yesterday: The species' behavior and body structure "continues the mystery of ant evolution," he said. The new species, he said, shows how remarkably specialized their behavior has become - "a strange mixture of the primitive and the derived," he called it.

Ant species are currently divided into two sub-families, and the Adetomyrma somehow share characteristics of both groups, so the distinctions between them are unclear - another indication that the entire ant evolutionary tree may need to be redrawn, Fisher said.

"The big question is, what has led to this amazingly successful development in evolution? It's so important because it will help us reconstruct the ancestral character of all the ants, and it will require us to rethink the entire process of ant evolution."